The present invention relates to an n-port coupler. A passive reciprocal loss-less four-port power divider provides inter alia a directional coupler if each port is matched. In the article "A directional coupler with very flat coupling", published in the periodical IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-26, No. 2, February 1978, pages 70-74, G. P. Riblet proposes a strip-line coupler having an assembly of four quarter-wave length long short-circuited stubs or four open-circuit half-wave stubs located at a distance of a quarter wave from the input ports of the coupler. Such a coupler is also known from the United Kingdom Patent Specification No. 1,582,285. In all of these cases the matching is realized outside the coupler.
In U.S. Pat. No. 4,127,832, as well as in the article "An Eigenadmittance Condition Applicable to Symmetrical Four-Port Circulators and Hybrids", by G. P. Riblet, published in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-26, No. 4, April 1978, pages 275-279, a strip-line hybrid coupler is suggested which for matching is provided with an assembly of four separated metal plates acting as capacitors and provided inside the coupler, the plates being connected to the four respective input ports by transmission lines. This comparatively compact coupler is, however, actually only adapted to one single frequency as is shown in FIG. 3 of the article.